The present invention relates to a process for producing both known 6-(substituted phenyl)-3(2H)pyridazinones, which are known to have valuable antifungal activity, and certain novel, closely related, compounds.
United Kingdom patent specification No. 1,533,010 (which corresponds to U.S. Pat. No. 4,052,395) discloses a series of 6-(substituted phenyl)-3(2H)pyridazinone compounds, which are said to have valuable fungicidal activity. Of the compounds disclosed in this specification, 6-(3,5-dichloro-4-methylphenyl)-3(2H)pyridazinone has been found to have particularly good activity, rendering it suitable for commercialisation; specifically, it has a broad antimicrobial spectrum and is very useful as an agricultural fungicide, particularly for the prevention of sheath blight.
The process disclosed in the aforementioned United Kingdom patent specification for the preparation of this compound comprises reacting 4-(3,5-dichloro-4-methylphenyl)-4-oxobutyric acid with hydrazine to prepare 6-(3,5-dichloro-4-methylphenyl)-4,5-dihydro-3(2H)pyridazinone, which is then dehydrogenated. This process is, however, not wholly satisfactory, in that the preparation of the starting material, 4-(3,5-dichloro-4-methylphenyl)-4-oxobutyric acid, is difficult, the process is expensive and control of the reaction is a problem as the dehydrogenation step is an exothermic reaction.
An alternative process for preparing this compound using the method disclosed in Japanese patent publication No. 428/81 comprises reacting 4-(3,5-dichloro-4-methylphenyl)-4-oxo-2-butenoic acid with methanol at 20.degree.-50.degree. C., in the presence of anhydrous potassium carbonate, to give potassium 4-(3,5-dichloro-4-methylphenyl)-2-methoxybutyrate, converting this compound to the corresponding free acid and then reacting the acid with hydrazine. This reference does not exemplify its process using 4-(3,5-dichloro-4-methylphenyl)-4-oxo-2-butenoic acid to produce the pyridazinone identified herein. The process of the reference, however, has the disadvantage that side reactions often occur producing various by-products, with the result that the purity of the product is low and, because of the nature of the impurities, purification is difficult.